Space · System
The Sun, stars and galaxies
Everything you can see with your eyes on a dark night, apart from a smudge or two, is inside one galaxy out of two trillion. Getting the sizes of things straight is most of the work.
Start here
There is a star you can see in the daytime.
It is 150 million kilometres away, it is 1.4 million kilometres across, and it is so bright you must never look at it directly.
What kind of object is the Sun?
A star, and an ordinary one. The Sun is a ball of hydrogen and helium about 1.4 million km across, fusing hydrogen in its core and pouring the energy out as light. It looks completely unlike the stars you see at night for one reason only: it is 270 000 times closer than the next nearest. Move the Sun to where Proxima Centauri is and it would be a faint dot you would need a dark night to notice.
A star is a ball of gas massive enough for its own gravity to squeeze the core until hydrogen nuclei fuse into helium, releasing energy. The Sun is one, and an unremarkable one: hotter and brighter than most stars, far cooler and dimmer than the largest.
A solar system is a star together with everything held in orbit around it — planets, moons, asteroids, comets and dust. Ours has one star. Most of the stars you can see at night have planets of their own, so most of them are the middle of a solar system too.
A galaxy is an enormous collection of stars — hundreds of billions of them — held together by gravity. Ours is the Milky Way, and the faint band of light it is named after is what the disc of it looks like seen edge-on from inside. Beyond that there are around two trillion other galaxies, and the whole lot together is the universe.
At the bench · five rungs of the ladder of scale
Every step is a huge jump, and the ladder does not stop.
Change a control to begin
Pick something and see how big it is, and how far its light has to travel — to reach you, or right across it if you are inside it. The bars are on a scale where each step is ten times the last — otherwise the first two would be invisible.
Commit first. Which of these is the correct order, smallest first?
What you are looking at
How big, and how far — the bars are drawn on a ten-times scale
What it is
—
—
How far light travels
—
—
Size
—
—
Stars in it
—
—
Key fact
A star is a ball of gas fusing hydrogen in its core; the Sun is one. A solar system is a star and everything orbiting it. A galaxy is hundreds of billions of stars bound together by gravity; ours is the Milky Way. The universe is every galaxy there is — around two trillion of them.
Think again
“The Sun is not a star — stars are the small twinkling things at night.”
The Sun is a star and the twinkling is not even a property of the star. It is caused by our own atmosphere: pockets of air at different temperatures bend the light this way and that on its way down, and a point of light appears to shimmer. Planets, which show a tiny disc rather than a point, average that shimmer out and shine steadily — which is one way of telling a planet from a star without a telescope.
“Stars are all about the same, and the brighter ones are the closer ones.”
Brightness in our sky mixes up two completely different things: how much light a star actually gives out, and how far away it is. Betelgeuse is around 500 light years away and looks bright because it is a red supergiant putting out something like 100 000 times the Sun’s light. Proxima Centauri is our nearest neighbour at 4.24 light years and is far too faint to see without a telescope. Untangling those two is one of the central problems of astronomy.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Order the scales
Put these in order from smallest to largest: galaxy, star, solar system, universe.
Rung 2 · The one that catches people
A student says the stars in the night sky are part of our solar system. What is wrong?
Rung 3 · Explain
Explain why the Sun looks so different from every other star, even though it is an ordinary one.
Rung 4 · Take it somewhere new
Andromeda is 2.5 million light years away. Explain what you are actually seeing when you look at it, and why astronomers call large telescopes time machines.
Key note
The Sun is an ordinary star: a ball of gas fusing hydrogen into helium in its core. The solar system is the Sun plus everything orbiting it, and it contains exactly one star. A galaxy is hundreds of billions of stars held together by gravity, and ours is the Milky Way. The universe holds around two trillion galaxies. The steps between these scales are not small — the nearest star is about 270 000 times further away than the Sun, and one galaxy is about a hundred thousand light years across.
Going further
The Milky Way is a barred spiral roughly 100 000 light years across, and the Sun sits about 26 000 light years from the centre, in a minor spiral arm, taking about 230 million years to complete one lap. The last time the Sun was in this part of its orbit, the first dinosaurs were appearing. At the centre is a black hole of about four million solar masses, photographed for the first time in 2022.
Almost everything you are made of was made inside a star. The hydrogen dates from the first few minutes of the universe; the carbon, oxygen, nitrogen and iron were fused in stellar cores and scattered by the deaths of stars that lived and died before the Sun formed. Heavier elements still — gold, platinum, uranium — appear to need the collision of two neutron stars. There is no ordinary chemistry that makes them.
Before this lesson
Connects to
At GCSE this becomes
- Stellar life cycles from nebula to main sequence to white dwarf or supernova, nuclear fusion as the source of a star’s energy, and red shift as evidence for an expanding universe.
Where to next
Ask Mr Badmus AI
Muddled about what sits inside what out there?
Never look at the Sun — not with your eyes, not through binoculars, not through a telescope, and not through sunglasses. It damages your sight in a way you cannot feel happening and cannot repair. Everything on this page is about the night sky.
The bench is a teaching model. Distances and sizes are accepted current figures rounded for display: the Sun 150 million km away and 1.39 million km across; Neptune’s orbit about 4.5 billion km from the Sun; Proxima Centauri 4.24 light years; the Milky Way about 100 000 light years across; Andromeda about 2.5 million light years away. Star counts for galaxies are estimates with wide error bars. The bars use a logarithmic scale so that quantities spanning eleven orders of magnitude can be shown together.
Lesson content © MrBadmusAI.